This invention relates to waveguide lasers, and more particularly to two or more parallel waveguide lasers of different effective length coupled to each other for phase locking with selection of the frequency, and suppression of other longitudinal modes, and with frequency tuning.
Coupled cavity lasers have been used for frequency selection in a wide variety of multiresonator design. In the case of semiconductor lasers, coupled cavity resonators have deserved special attention since they offer the capability of stable single mode operation over a wide range of modulation conditions, frequency tunability and output power control. The effect of coupling two cavities in these devices is to interferometrically modulate the gain and loss of the composite resonator resulting in enhancement of lasing action at those frequencies in which the resonances of the two cavities reinforce each other. This results in frequency selectivity.
An additional improvement is to provide a separate electrical contact for the cavities to allow a desirable electronic control of the output spectrum by changing the gain and index of refraction in each cavity. Such a structure has been implemented in the cleaved-coupled cavity laser on which intensive analytical and experimental efforts have been invested. See "Stabilization of Single Frequency Operation of Coupled-Cavity Lasers" by Charles H. Henry, et al., IEEE J. Quantum Electron., Vol. QE-20, pp 733-744, 1984, and papers cited therein. However, the performance of such lasers depends in a critical way on the dimensions of the air gap between cavities. That gap has to be adjusted with tolerances of the order of a small fraction of the lasing wavelength. This creates a significant problem in the fabrication process of the device.
It would be desirable to achieve the frequency selectivity in lateral coupled laser structures, like phase-locked laser arrays, to avoid the need to adjust an air gap between cavities. However, most of the analyses of phase-locked arrays have been related to the lateral modes of the structure (supermodes), and no attention has been paid to the longitudinal mode selection that can be achieved by phase-locking parallel lasers of different effective optical lengths.
The present inventors recently performed a first analysis of the longitudinal resonances of two laterally coupled semiconductor lasers, and showed that when the reflectivity of the two mirrors differs, thereby making their effective length different, the two supermodes are coupled in a way similar to that in which two cavity modes are coupled in a cleaved-coupled cavity laser. These results are enhanced in the case of two cavities of different physical lengths.